1. Field of the Invention
The present invention relates to tyres for motor vehicles, particularly directional tyres for use on snow-covered ground.
2. Description of the Related Art
A tyre, in its most general form, comprises: a carcass structure including a central crown portion and two axially opposing sidewalls terminating in a pair of beads for securing the tyre to a wheel rim, a belt structure associated coaxially with the carcass structure and a tread band extending coaxially around the belt structure.
The tread band is generally moulded with a relief pattern formed by a plurality of longitudinal and transverse grooves which, in combination, give rise to a plurality of blocks distributed according to different types of configurations, for example along a central area extending across the equatorial plane and in at least two shoulder areas extending in axially opposed positions on either side of said central area.
Of particular importance for the purposes of travel on a snow-covered surface is the presence, in the blocks, of suitable sipes, in other words of a closely spaced set of incisions or small grooves which are circumferentially distributed and orientated essentially transversely with respect to the rolling direction. The function of said sipes is essentially to collect and retain the snow in an efficient way, since it has been shown to be generally convenient to exploit the friction of snow on snow to ensure a good grip of a tyre on snow-covered ground.
One type of winter tyre marketed by the Applicant has, in the tread band, a pair of rectilinear circumferential grooves, arranged symmetrically with respect to the equatorial plane of the tyre and spaced apart from each other by a distance essentially equal to half of the total width of the tread band.
Said circumferential grooves are combined with two sets of transverse grooves, each of which extends with a progressively increasing inclination from the outer edge of the tread band to a point on the equatorial plane.
The transverse grooves of said two sets converge symmetrically on the equatorial plane of the tyre in a predetermined preferred rolling direction, and have their vertices interleaved with each other alternately along the perimetric extension of the tread band in correspondence of the equatorial plane.
In said tread band, the configuration of the transverse grooves is such as to promote good characteristics of traction because of the mutual convergence of the grooves in the rolling direction.
Moreover, the circumferential rectilinear grooves enable water to be removed efficiently from the footprint while the tyre is running on a wet surface, thus preventing the onset of the well-known and dangerous phenomenon of aquaplaning.
The traction and the overall behavior in travel on a snow-covered surface are improved by the provision of closely spaced sipes in the blocks obtained by the intersection of the longitudinal grooves with the transverse ones, and by suitable rectilinear grooves, associated with the transverse grooves, in the space delimited by the two circumferential grooves, each of said rectilinear grooves extending in the circumferential direction for a short distance from the corresponding transverse groove.
Patent EP-773,116, in the name of the same Applicant, discloses a tyre for snow-covered surfaces, provided with a tread band in which the circumferential grooves are formed by a sequence of portions orientated obliquely with respect to the circumferential direction of the tyre. Said oblique portions converge symmetrically towards the equatorial plane according to a direction opposite to the direction of convergence of the transverse grooves.
Each of the oblique portions of the circumferential grooves extends between two consecutive transverse grooves and delimits respectively the opposing circumferential edges of a centre block and of a shoulder block.
In this solution, each of the centre and shoulder blocks has a corner projecting into the inside of the corresponding circumferential groove with respect to the opposite corner of the block immediately following it.
In this situation, the projecting corners of the centre blocks act as teeth which provide the grip on snow in traction, while the projecting corners of the shoulder blocks produce this effect during braking.
In order to increase the grip on snow, said tyre also has a groove of trapezoidal shape extending partially between two adjacent blocks belonging to the same row of centre blocks.
Said groove passes through the transverse groove between the two centre blocks and has an inner circumferential edge and an outer circumferential edge which diverge from each other at angles preferably comprised from 3° to 15°, in the opposite direction to the rolling direction.
However, since the tread pattern for a tyre suitable for running on snow-covered ground has a considerable number of blocks, it inevitably causes a certain degree of noise when running on dry roads because of the successive impacts of the frontal profiles of the blocks on the ground.
U.S. Pat. No. 5,759,313 relates to a pneumatic tyre permitting a vehicle to travel excellently both in the on-road condition and in the off-road condition without changing its tyres. To achieve such an object, the document discloses a tyre, wherein sub-grooves extending at least in the widthwise direction of the tyre are arranged in the circumferential direction thereof at a predetermined pitch in a center portion of a tread surface and in shoulder portions thereof on both sides of the center portion. Both the width and depth of the sub-grooves in both of the shoulder portions are set larger than those of the sub-grooves in the center portion. A ratio of the area of the sub-grooves in both of the shoulder portions to that of the tread surface is set larger than a ratio of the area of the sub-grooves in the center portion to that of the tread surface.
In considering the cited prior art, the Applicant has perceived the necessity of providing a tyre for snow-covered ground which can simultaneously ensure a good grip on said type of ground, a satisfactory noiseness when running on dry roads, and a high wear-resistance of the tread blocks.